Solar collector

ABSTRACT

A solar collector, includes a glass sheet provided with a fired metal frit; a metal frame or another glass sheet provided with a fired metal frit and a metal frame; a brazed seal between the metal frit or frits and the metal frame; an absorber and pipes in which a heat-transfer fluid circulates, the pipes being in contact with the absorber, and the absorber and the pipes being placed between the glass sheet and the metal frame or between the two glass sheets. The invention provides a solar collector which is compact and simple and improves solar radiation transmission.

The invention relates to a solar collector.

Solar collectors absorb the heat from solar radiation by virtue of an absorber. A heat-transfer fluid circulates in heat collector pipes fixed to the absorber. The pipes enable the heat to be transported to the user and make it possible to keep the absorber at a reasonable temperature. Such a collector may be used for example to heat water for domestic applications, to supply thermal energy to a refrigeration unit for producing conditioned air, to desalinate sea water or to purify water for the purpose of supplying drinking water, or else to dry materials in an industrial plant.

Document GB-A-2 261 247 discloses a solar collector comprising a multiple glazing unit beneath which an absorber is placed. The multiple glazing unit comprises at least one pair of glass sheets spaced apart by a metal spacer welded to the glass sheets via strips of conductive enamel.

One drawback of this solar collector is that the absorber and the pipes in which the heat-transfer fluid circulates are placed beneath the multiple glazing unit. The solar collector therefore has a large thickness, which causes handling problems when fitting it. Moreover, it is necessary to add a thermally insulating coating beneath the absorber and the pipes so as to minimize the heat losses. This complicates the solar collector and further increases the thickness of the solar collector. In addition, the solar radiation must pass through at least two glass sheets in order to reach the absorber, thereby reducing the transmission of the solar radiation.

There is therefore a need for a solar collector which is compact and simple and improves the transmission of solar radiation to the absorber.

To do this, the invention provides a solar collector, comprising:

a glass sheet provided with a fired metal frit;

a metal frame or another glass sheet provided with a fired metal frit and a metal frame;

a brazed seal between the metal frit or frits and the metal frame;

an absorber and pipes in which a heat-transfer fluid circulates, the pipes being in contact with the absorber, and the absorber and the pipes being placed between the glass sheet and the metal frame or between the two glass sheets.

According to another feature, the solar collector comprises a single glass sheet and a metal frame, the metal frame being provided with a bottom and the free edge of the metal frame being brazed to the metal frit of the glass sheet.

According to another feature, the solar collector comprises two glass sheets and a metal frame, the edges of the metal frame being brazed to the metal frits of each of the two glass sheets.

According to another feature, the metal frame comprises a wall offset with respect to the brazed seal and/or a wall connected to the brazed seal by a low thermal conductivity material.

According to another feature, the glass sheet(s) is (are) tempered.

According to another feature, the brazing alloy has a melting point between 100° C. and 350° C.

According to another feature, the brazing alloy is the alloy Pb_(93.5)Sn₅Ag_(1.5).

According to another feature, the fired metal frit comprises between 50% and 95% by weight of silver particles, the rest being a vitreous binder comprising SiO₂, Bi₂O₃, Na₂O and ZnO.

According to another feature, the solar collector is under a vacuum.

According to another feature, the glass sheet is made of extra-clear glass.

According to another feature, the glass sheet is provided with an antireflection coating.

According to another feature, each glass sheet is joined to an additional glass sheet via a polymeric interlayer in order to form a laminated glazing unit.

Another object of the invention is to provide a solar collector comprising:

a glass sheet;

a metal frame or another glass sheet and a metal frame;

a seal between the glass sheet or sheets and the metal frame,

the metal frame comprising a wall offset with respect to the brazed seal and/or a wall connected to the brazed seal by a low thermal conductivity material.

Other features and advantages of the invention will now be described in conjunction with the drawings in which:

FIG. 1 shows a cross-sectional view of a solar collector according to a first embodiment of the invention;

FIG. 2 shows a cross-sectional view of a solar collector according to a second embodiment of the invention; and

FIG. 3 shows a cross-sectional view of a solar collector according to a third embodiment of the invention.

The reference numbers that are the same on the various figures represent identical or similar elements.

The invention relates to a solar collector comprising a glass sheet provided with a fired metal frit. The solar collector also comprises another glass sheet provided with a fired metal frit and/or a metal frame.

A brazed seal is produced between the metal frit or frits and the metal frame or between the metal frits of the two glass sheets. Thus, the solar collector is hermetically sealed, thereby making it possible in particular to maintain a vacuum in the solar collector.

The solar collector also comprises an absorber and pipes in which a heat-transfer fluid circulates. The pipes are in contact with the absorber so as to maximize the heat exchange between the absorber and the heat-transfer fluid. In addition, the absorber and the pipes are placed between the glass sheet and the metal frame, or between the two glass sheets.

Thus, the solar collector is compact since the absorber and the pipes are integrated between the glass sheet and the metal frame, or between the two glass sheets. In addition, the solar collector is simple, as it avoids the use of an additional insulating coating. The solar radiation has only a single glass sheet to pass through in order to reach the absorber. This makes it possible to improve the transmission of the solar radiation to the absorber.

FIG. 1 shows a cross-sectional view of a solar collector according to a first embodiment of the invention.

In this embodiment, the solar collector comprises a glass sheet 1 and a metal frame 2 sealed onto the glass sheet 1. The seal is produced by brazing with the aid of a brazing alloy 4 via a metal frit 3 deposited on the glass sheet 1. Such a seal between the glass and the metal is mechanically strong and remains impermeable. This seal is particularly advantageous when the solar collector is under a vacuum, since it prevents the vacuum from deteriorating over the course of time.

The metal frit 3 is deposited on the perimeter of one face of the glass sheet 1, preferably by screen printing. Deposition by screen printing is in fact simpler than thin-film deposition in the context of industrialization.

The metal frit 3 is dried at 80° C. The glass sheet 1 provided with the metal frit 3 are then heated to a temperature between 400° C. and 700° C. so as to fire the metal frit 3. This firing temperature makes it possible for the glass sheet 1 not to be damaged. The glass sheet 1 provided with the fired metal frit is then cooled to room temperature.

If the glass sheet 1 of the solar collector is made of tempered glass, the firing of the metal frit 3 is carried out during the thermal tempering of the glass sheet. The firing temperature of the frit is then preferably above 600° C. and the cooling takes place with the aid of a plurality of nozzles injecting compressed air close to said glass sheet. The final surface stress in the glass is then for example 120 MPa, for a glass 4 mm in thickness, and the silver frit is fired.

The solar collector may be intended to be installed on a roof, for example to heat water for domestic applications. The fact that the glass sheet 1 is made of tempered glass enables the mechanical properties of the glass to be increased, so that the glass sheet 1 is more resistant to foul weather, for example resistant to hailstones, and to the mechanical stresses induced by atmospheric pressure on the solar collector and by thermal expansion of the glass sheet of the solar collector.

Note that the step of depositing the metal frit by screen printing on the perimeter of one face of the glass sheet 1 is particularly well integrated into an industrial thermal tempering line.

The fired metal frit 3 comprises between 50% and 95% by weight of silver particles, the rest being a vitreous binder. Thus the fired metal frit consists for example of 94% by weight of silver particles and 6% by weight of vitreous binder comprising SiO₂, Bi₂O₃, Na₂O and ZnO.

The metal frit 3 adheres perfectly well to the glass sheet 1 and is thus particularly well suited to brazing with another metallic element, namely here the metal frame, so as to form a hermetic seal.

The metal frame 2 comprises a bottom 20 and a free edge 21 intended to be brazed to the metal frit 3. Preferably, the free edge 21 and the metal frit 3 are descaled before brazing, thereby enabling a better seal to be obtained.

The bottom 20 of the metal frame is also metallic. This makes manufacture easier since the edge 21 and the bottom 20 may be made as one piece, for example by deep drawing, or else they may be conventionally welded.

The solar collector also comprises an absorber 5 and pipes 6 in contact with the absorber. The absorber 5 is designed to absorb the solar radiation transmitted through the glass sheet 1. For example, the absorber 5 is a metal plate covered with a low-emissivity coating. The metal allows good solar radiation absorption, while the low-emissivity coating ensures that the least possible amount of solar radiation is reemitted to the outside of the solar collector.

A heat-transfer fluid 7, for example water, circulates in the pipes 6. The heat coming from the solar radiation that passes through the glass sheet 1 is transmitted from the absorber 5 to the pipes 6 and then to the heat-transfer fluid 7.

The absorber 5 and the pipes 6 are placed inside the metal frame 2. The fired metal frit 3 of the glass sheet 1 is then brazed onto the free edge 21 of the metal frame 2 with the aid of a brazing alloy so as to seal the solar collector. The absorber 5 and the pipes 6 are kept at a certain distance from the glass sheets, for example using spacers (not shown). These spacers also make it possible to withstand the pressure difference between the external air and the internal vacuum.

The brazing alloy used preferably has a melting point between 100° C. and 350° C. If the glass sheet 1 is made of tempered glass, this relatively low melting point prevents the glass from becoming detempered. Moreover, it prevents the low-emissivity properties of the absorber 5 from deteriorating and limits the mechanical stresses induced by the difference in thermal expansion coefficient between the glass and the metal.

If the solar collector is to be under a vacuum, this is created between 100 and 300° C., after the brazing step. This is because creating a vacuum is more effective if it is done at high temperature, thereby accelerating the desorption and increasing the pressure inside the solar collector. Creating the vacuum within the solar collector provides excellent insulation between the absorber 5 and the external medium by cutting out convective and conductive heat transfer in the internal air. This greatly improves the efficiency of the solar collector obtained.

Preferably, a melting point of the brazing alloy between 250 and 350° C. is a good compromise between the need to heat when creating the vacuum without remelting the brazing alloy and the need not to heat the glass too much so as not to detemper it.

The brazing alloy is for example the alloy Pb_(93.5)Sn₅Ag_(1.5), which has a melting point of 300° C.

The glass sheet 1 may also comprise a low-emissivity coating on its surface, preferably on the inside of the solar collector so that it is not degraded due to foul weather. This low-emissivity coating may be deposited on the glass sheet before or after the metal frit has been deposited.

The glass of the glass sheet 1 may be an extra-clear glass so as to minimize the absorption of solar radiation and thus maximize its energy transmission. The glass sheet 1 may also be provided with an antireflection coating on its external face.

In an installation on a building roof, the glass sheet 1 is turned to the outside of the building, whereas the bottom 20 of the metal frame 2 is turned toward the building.

This first embodiment has the advantage over the second and third embodiments, which will be described below, of having only a single brazed seal, thereby limiting the risk of leakage.

FIG. 2 shows a cross-sectional view of a solar collector according to a second embodiment of the invention.

In this embodiment, the solar collector comprises two glass sheets 1 and a bottomless metal frame 8. The bottomless metal frame 8 serves as a spacer between the two glass sheets 1. The edges 81 of the metal frame 8 are each brazed to the fired metal frit 3 of one of the glass sheets 1 so as to seal the solar collector.

The two glass sheets 1 may be made of tempered glass. As a variant, only one of the two glass sheets—the one through which the sun's rays pass directly—is made of tempered glass.

In an installation on a building roof, one or other of the two glass sheets 1 may be turned toward the outside of the building, the other glass sheet being turned toward the building.

Moreover, all that was stated in respect of the first embodiment remains valid in respect of this second embodiment.

This second embodiment has the advantage of making the structure symmetrical, thereby preventing the collector from bending when it is subjected to temperature variations caused by the difference in thermal expansion coefficient between the glass and the metal, for example either when creating the vacuum or during use.

FIG. 3 shows a cross-sectional view of a solar collector according to a third embodiment of the invention.

This embodiment is a variant of the second embodiment. Only the metal frame 8 differs from the second embodiment. The metal frame 8 consists of a wall 80 approximately perpendicular to the glass sheets 1 and of turned-over edges 81 approximately parallel to the glass sheets 1. The edges 81 and the wall 80 may be made of one piece, for example by deep drawing, or else the edges 81 may be attached to the wall 80.

The third embodiment includes a metal frame especially designed to minimize heating of the brazed seal during use of the solar collector.

In fact the pipes 6 of the solar collector pass through the wall 80 of the metal frame in a sealed manner so as to enter and leave the solar collector. The pipes are at a high temperature, for example around 80° C. in the case of domestic applications and around 170° C. in the case of refrigeration applications with two-stage absorption machines. The wall 80 will therefore be heated by the pipes 6. It is advantageous to limit heat exchange between the wall 80 and the brazed seal 4 so as not to damage the latter, so that it can remain vacuum-tight as long as possible, so as to guarantee the longevity of the solar collector.

A first solution for limiting heat exchange between the wall 80 and the brazed seal 4 is for the edges 81, and optionally the wall 80, of the metal frame 8 to have a low thermal conductivity, for example by being made of a low thermal conductivity material and/or by having a small thickness. Thus, the edges 81 and optionally the wall 80 preferably have a thermal conductivity of less than 20 W/m/K, more preferably less than 15 W/m/K and ideally less than 1 W/m/K. For example, they are therefore made of stainless steel or else of the alloy referred to by the trade mark Kovar®, which has a thermal conductivity of 17 W/m/K. Likewise, the thickness of the edges 81 and optionally of the wall 80 is preferably less than 1 mm, more preferably less than 0.5 mm.

A second solution for limiting heat exchange between the wall 80 and the brazed seal 4 is for the wall 80 of the metal frame 8 is to offset the wall 80 by a distance of at least 1 cm, preferably 2 cm, from the brazed seal 4 with the aid of the edges 81.

The first and second solutions may be combined on the same metal frame so as to further reduce heat exchange between the wall 80 and the brazed seal 4.

Thus, the second embodiment provides a compact solar collector whereas the third embodiment enables the performance of the solar collector to be optimized.

The action of offsetting the wall of the metal frame with respect to the glass/metal seal and/or of reducing the thermal conductivity of the edges of the metal frame, by choosing a suitable material and/or by reducing the thickness of the edges and optionally of the wall of the metal frame, may be applied to a solar collector in which the glass sheet or sheets are sealed to the metal frame by another conventional technique, namely a technique other than by brazing via a metal frit.

The action of offsetting the wall of the metal frame with respect to the glass/metal seal and/or of reducing the thermal conductivity of the edges of the metal frame, by choosing a suitable material and/or by reducing the thickness of the edges and optionally of the wall of the metal frame, may be applied to the embodiment shown in FIG. 1.

The spacers (not shown), which it possible to withstand the pressure difference between the external air and the internal vacuum, compensate for the loss of compressive strength of the wall 8 due to its small thickness and/or the fact that it is offset, and thus prevent it from bowing under the effect of the vacuum, so as to maintain the structure of the collector.

In the three embodiments, each glass sheet 1 may be combined with an additional glass sheet via a polymeric interlayer so as to form a laminated glazing unit. This results in greater personal safety vis à vis the risk of an implosion inherent in any glass system under vacuum. 

1. A solar collector, comprising: a glass sheet provided with a fired metal frit; a metal frame or another glass sheet provided with a fired metal frit and a metal frame; a brazed seal between the metal frit or frits and the metal frame; an absorber and pipes in which a heat-transfer fluid circulates, the pipes being in contact with the absorber, and the absorber and the pipes being placed between the glass sheet and the metal frame or between the two glass sheets.
 2. The solar collector as claimed in claim 1, comprising a single glass sheet and a metal frame, the metal frame being provided with a bottom and a free edge of the metal frame being brazed to the metal frit of the glass sheet.
 3. The solar collector as claimed in claim 1, comprising two glass sheets and a metal frame, the edges of the metal frame being brazed to the metal frits of each of the two glass sheets.
 4. The solar collector as claimed in claim 1, wherein the metal frame comprises a wall offset with respect to the brazed seal and/or a wall connected to the brazed seal by a low thermal conductivity material.
 5. The solar collector as claimed in claim 1, wherein the glass sheet is, or the glass sheet and the other glass sheet are, tempered.
 6. The solar collector as claimed in claim 1, wherein a brazing alloy of the brazed seal has a melting point between 100° C. and 350° C.
 7. The solar collector as claimed in claim 6, wherein the brazing alloy is the alloy Pb_(93.5)Sn₅Ag_(1.5).
 8. The solar collector as claimed in claim 1, wherein the fired metal frit comprises between 50% and 95% by weight of silver particles, the rest being a vitreous binder comprising SiO₂, Bi₂O₃, Na₂O and ZnO.
 9. The solar collector as claimed in claim 1, the solar collector being under a vacuum.
 10. The solar collector as claimed in claim 1, wherein the glass sheet is made of extra-clear glass.
 11. The solar collector as claimed in claim 1, wherein the glass sheet is provided with an antireflection coating.
 12. The solar collector as claimed in claim 1, wherein each glass sheet is joined to an additional glass sheet via a polymeric interlayer in order to form a laminated glazing unit. 